EFFECT OF PARASITIC FUNGI ON THE FORM OF HOST-PLANT. 31 



Somewhat doubtful cases are the outgrowths resembling aerial 

 roots which arise on Laurus canaricnsis attacked by Exohasidimn 

 lauri. Geyler, their discoverer, regarded them as deformed stem- 

 shoots, but they resemble rather the galls of the alpine-rose. 



§ 6. EFFECT OF PARASITIC FUNGI ON CELL-CONTENTS. 



The most common and, at the same time, most apparent 

 effect of parasitic fungi in this direction, is the stimulation to 

 cell-division and cell-multiplication. This occurs chiefly in 

 young tissues, or in those still in process of growth, and gives 

 rise to numerous peculiar outgrowths and swellings, some of 

 which have already been referred to. 



The parenchyma of mature tissues may also exhibit secondary 

 cell-division, when under the influence of a parasitic fungus. 

 This I found to be the case in leaf- 

 petioles of Umbellifer^e attacked by 

 Pwtomyces macrosponis (Fig. 9). The 

 epidermis and vascular bundles are never 

 disturbed, but the intervening tissues are 

 permeated by an intercellular mycelium, 

 which causes the cells to divide into a 

 large number of delicate-walled chambers, 



, . . Fig. 9.— Secondary cell-division 



all COntammg nuclei smaller than those in parenchyma of petiole of ^<;6ro- 



. . 2wdium as a. result of Protomi/ces 

 of neighbourmg undivided cells. The maavsporus. The nuclei of the 



'- . . new cells are much smaller than 



same thing is observed in plants of those of the primary cell. (Com- 



rr- 7 J , • -U 1 -^ 1 1 TT ,- pare Fig. 47.) (v. Tubeuf del.) 



Viola odorata inhabited by Urocystis 



violae ; the mature parenchymatous cells become divided up 

 by means of delicate walls running in various directions into 

 numerous chambers or secondary cells, which Wakker in 

 describing has named " nutritive tissue." ^ This new tissue 

 remains permanently in attacks of Protom.ifces, but with Uro- 

 cystis it is almost completely used up during the formation 

 of spores. In some diseases caused by Exoasceae, a similar 

 secondary cell-division takes place ; for example, in the sub- 

 epidermal parenchyma of leaves of poplar with Tcqohrina 

 aurca (Fig. 63). 



An interesting observation was made by Rosen ^ on the direct 



^Wakker, " Untersuchungen. " Pringshetm's Jahrbuch, 1892. 

 -Rosen, Beitriirje z. Kenntniss d. Pflanzenzelle. Habil.-Schrift, 1892. 



